A system of this type is known from IEEE Technical Digest on Optical Amplifiers and their Applications, Monterey, August 1990, pp 232-235 (WB1). The system described there is a pure distribution system for television signals. A large number of subscribers is connected by means of a multistar fiber-optic network to a television center. Fiber-optic amplifiers are present between successive branch points of the fiber-optic network, each of which consists of an erbium-doped length of fiber and a pump source. A frequency band containing the television signals to be transmitted is converted into an optical signal with a wavelength of 1552 nm, and the optical system is transmitted via the fiber-optic network to the subscribers, where it is amplified in the fiber-optic amplifiers.
In many applications, there is the additional requirement for the transmission, in addition to the television signals, of signals of bidirectional services (dialog services), such as, e.g., telephone and data transmission services, between the center and the subscribers and vice versa.
An optical communications system that can transmit not only television signals but also signals of bidirectional services between a center and subscribers is known from German patent application DE-A1 39 07 495. In this, the center is connected by means of an optical waveguide with a front-end device containing a star coupler, from which subscriber-assigned optical waveguides lead to a group of subscribers. These signals to be transmitted from the center to the subscribers are converted as a frequency band into an optical signal with a first wavelength, and this optical signal is transmitted to the subscribers. The signals to be transmitted from the subscribers to the center are converted into signals with subscriber-assigned frequencies, and these are transmitted as optical signals with a second wavelength via the star coupler to the center. The number of subscribers that can be serviced with an optical transmission system of this type is limited to a relatively small number in such a system even if, as is mentioned there, optical amplifiers are present in the star couplers.
It is therefore the task of the invention to indicate an optical communications system of the type mentioned above that is suitable for a larger number of subscribers.